The parameter to SDL_AppEvent() should be non-const

This allows functions like SDL_ConvertEventToRenderCoordinates() to work without having to copy the event.

Fixes https://github.com/libsdl-org/SDL/issues/10691

test/testcamera.c

@@ -216,7 +216,7 @@ static int FlipCamera(void)
     return SDL_APP_CONTINUE;
 }
 
-SDL_AppResult SDL_AppEvent(void *appstate, const SDL_Event *event)
+SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
 {
     switch (event->type) {
         case SDL_EVENT_KEY_DOWN: {